The present invention relates to an image forming apparatus employing an electrostatic transfer process such as an electrophotographic copying machine and an electrostatic recording apparatus.
For charging an image forming member such as a photoreceptor drum, generally a corona charger has hitherto been used wherein high voltage is impressed upon a discharge wire and thereby a strong electric field is generated around the discharge wire for gaseous discharge. The image forming member is charged when electric charge ions generated in the process of gaseous discharge are adsorbed on the image forming member.
A corona charger used in the conventional image forming apparatus mentioned above has an advantage that an image forming member is not damaged in the process of charging thereon because the charger does not come into mechanical contact with the image forming member. The corona charger, however, has a disadvantage, due to high voltage used therein, that there is a risk of an electric shock or electric leakage and also ozone generated in the course of gaseous discharge is harmful to human bodies and the ozone shortens a life of the image forming member. Further, charging voltage by means of a corona charger is sharply influenced by temperature and humidity to be unstable, and noise is caused by high voltage in the corona charger, which is a serious disadvantage on the occasion where an electrophotographic image forming apparatus is utilized as a terminal unit for communication or an information processing apparatus.
These many disadvantages of a corona charger are caused by gaseous discharge necessary for charging.
Therefore, there are disclosed in Japanese Patent Publication Open to Public Inspection Nos. 133569/1984, 21873/1992 and 116674/1992 (hereinafter referred to as Japanese Patent O.P.I. Publication) the charging devices wherein magnetic particles are adsorbed on a cylindrical conveying carrier which is a charging roller holding therein magnetic objects for forming a magnetic brush, and the magnetic brush rubs the surface of the image forming member for charging it, as a charging device capable of charging the image forming member without conducting high voltage gaseous discharge carried out in a corona discharge and without giving any mechanical damages on the image forming member.
However, even the charging devices disclosed in the aforementioned Japanese Patent O.P.I. Publications have a problem in which an image forming member can not be charged uniformly, completely and stably. The problem will be described as follows:
In the transfer region, magnetic particles on the surface of the cylindrical magnetic particle conveying carrier are formed into a chain shape, and charging is conducted through this chain-shaped magnetic brush. Therefore, the image forming member is locally over-charged, and dielectric breakdown and uneven charging are caused on the image forming member.
As a specific example, charging is conducted through this magnetic brush. In this case, at a position where the magnetic particle conveying carrier and the image forming member are located most closely, the magnetic particles are compressed, so that the resistance of the magnetic brush is lowered. Accordingly, a current flows at a position on the image forming member where the resistance is low, so that the voltage of the entire magnetic brush is lowered.
Consequently, because of a spot-shaped defective portion 10d shown in FIG. 7, an entire stripe-shaped region R on the image forming member 10 where the magnetic brush comes into contact is defectively discharged, and in some cases, the power source for bias voltage is damaged.